1. Field of the Invention
The present invention relates to a method for producing a soot body by depositing glass particles on a glass rod while reciprocating the glass rod and glass particle synthesizing burners relatively. More particularly, the present invention relates to a method for producing a soot body which has a long effective portion and short taper portions (ineffective portions) formed at both ends.
2. Description of the Related Art
There is a method for producing a large soot body at high deposition rate in which a plurality of glass particle synthesizing burners are arranged at regular intervals to be opposed to a glass rod within a reaction vessel, and glass particles synthesized by the burners are deposited in layers on the surface of the glass rod by reciprocating relatively a burner array and the glass rod while rotating the glass rod.
In this method for producing the soot body, the main subjects involve reducing the variation in an outer diameter in a longitudinal direction of the soot body from a viewpoint of improvement of quality, and making a length of taper portions (ineffective portion) formed at both ends of soot body as short as possible from a viewpoint of productivity. Various methods have been offered.
For example, there is a method in which a turn-back position of reciprocating movement between the glass rod and the burners is shifted in a predetermined direction at every turn-back, and shifted in a reverse direction if the turn-back position reaches a predetermined position to be back to an initial position of the reciprocating movement (Japanese Patent No. 2612949). At the turn-back position, the deposition time of glass particles is substantially lengthened, and a degree of touching of burner flame against the soot body is changed, thereby resulting in the variation of the outer diameter in the turn-back position. By dispersing the turn-back position over the entire soot body as mentioned above, it is possible to equalize the deposition amount of glass particles in the longitudinal direction and reduce the variation in the outer diameter.
In this method, since the turn-back position is shifted, an area where glass particles synthesized by the burners arranged at both ends of the burner array are deposited is lengthened, so that the ineffective portion is longer than when the turn-back position is not shifted. Simultaneously, the portion that ought to have been an effective portion becomes the ineffective portion, resulting in a problem that the length of the effective portion is shorter, and the yield is decreased.
The soot body is vitrified, finished through a flame polishing process, and drawn to have an optical fiber. To remove a portion of larger outer diameter in the ineffective portion of soot body, it is required that the glass of larger outer diameter portion is melted after vitrification, and torn off, thereby taking more processes and higher costs.
Alternatively, the optical fiber obtained from the ineffective portion is discarded after the optical fiber is formed from the soot body in a drawing process. However, if the ineffective portion is long, it takes a lot of time to start drawing the effective portion in the drawing process, thereby taking higher costs.